Sliding sash windows comprising two or more window panels, at least one of which can slide in a vertical direction in use to open and close the window, have been in long term and widespread use. During sliding movement, it is necessary for the user to overcome the frictional resistance to movement caused by engagement of the sliding window panel or sliding sash with the channel in which it moves, and also to support the weight of the sliding sash.
To assist the user in moving the sliding sash, it is known to use a pair of counterweights suspended by respective pulleys within the window frame, one weight to each side of the sliding sash, which counterweights are intended closely to match the weight of the sliding sash. Notwithstanding that the user still has to overcome the frictional resistance to movement, that resistance is often relatively small so that the use of counterweights allows the user more easily to lower (and in particular raise) the sliding sash.
Counterweights have to large extent been replaced by spring balances which utilise one or more springs, and which generate a spring force to seek to counterbalance some or all of the weight of the sliding sash.
Spring balances use a tension spring which is connected at its top to the frame and at its bottom to the sliding sash. Downwards movement of the sliding sash causes the tension spring to extend, providing a return force which will support some or all of the weight of the sliding sash and also assist the user when moving the sliding sash upwardly. However, since the return force provided by a tension spring varies with extension of the spring, a second spring is used so that the resultant return force provided by both springs is more uniform over the range of movement of the sliding sash.
The second spring is typically a helical torsion spring which is held at one end and with the “free” end being forced to rotate as the sliding sash is moved. Rotation of the free end of the torsion spring is usually effected by a spiral rod which is connected to the sliding sash and which is caused to move through a follower bush carried by the free end of the torsion spring, the follower bush converting the relative axial movement of the spiral rod into rotation of the free end of the torsion spring. Such a spring balance is disclosed for example in GB patent 819,094.
The return force can be made more uniform throughout the range of movement of the sliding sash by adjusting the pitch of the spiral rod along its length, i.e. the rate of rotation of the torsion spring can be made dependent upon the extension of the tension spring.
Spring balances are usually adjustable, i.e. the amount of tension in the tension spring and/or the amount of torsion in the torsion spring can be adjusted. This allows the return force provided by the spring balance to be adjusted to match the weight of the particular sliding sash to which it is fitted, and also to ensure that the return force is maintained as desired over the lifetime of the spring balance, during which lifetime the return force provided by the tension spring and/or the torsion spring may reduce. The adjustment is typically effected by varying the preload upon the torsion spring.
The spring balance of GB 819,094, for example, is adjustable by way of rotating the “free” end of the torsion spring, i.e. the bottom end of the torsion spring, relative to the tension spring, by way of the spiral rod. In practice, this requires the bottom of the spring balance to be disconnected from the sliding sash so that the necessary access can be gained to rotate the spiral rod.
Whilst many different types of spring balance are known, employing many different adjustment methods, most require access to be gained to the bottom of the spiral rod, which requires disconnection of the bottom of the spring balance (at least) from the sliding sash, and in some cases requires removal of the entire sliding sash.
When such a spring balance is sold to the window assembler or installer the return force is usually set to its minimum so that the assembler or installer can increase the return force to that necessary to match the weight of the particular window being fitted.
Also, as the return force gradually reduces during the lifetime of the spring balance further adjustment must be effected to increase the return force when required. This adjustment is a specialised task usually requiring specialist assistance i.e. the person requiring adjustment of a fitted spring balance will seldom have the ability or confidence to disconnect or remove the spring balance or sash to effect adjustment, nor have the knowledge of how to adjust the spring balance in any event. Adjustment is made more difficult because both spring balances in each pair must be adjusted together to ensure that the return force is substantially balanced to each side of the window.
The requirement for a specialist to adjust the spring balances during their lifetime is inconvenient at the very least. Also, adjustment of the spring balances during their lifetime and also during initial assembly of the window is a time consuming task since as above indicated the spring balance must usually be disconnected from the sliding sash to effect adjustment, so that the adjustment is often undertaken in several stages, somewhat on a “trial and error” basis, with the spring balance being re-connected after each trial to learn whether the adjustment is correct.
Even if the manufacturer of the spring balances seeks to avoid the need for adjustment during assembly by setting the desired preload for a particular sliding sash, this is not always successful in practice, and it is believed that around 80% of spring balances require adjustment during window assembly, and most spring balances will require subsequent adjustment during their lifetime.
In addition, most spring balances allow adjustment in one direction only, i.e. they employ a ratchet mechanism or the like allowing the return force to be gradually increased, both initially to match the weight of a particular sliding sash to which the spring balance is to be fitted, and also to counteract any reduction in return force over the lifetime of the spring balance. In the event that the spring balance is over-adjusted, i.e. the return force is made too great, it is often necessary completely to dismantle the spring balance in order to reduce the spring force, e.g. to set the return force at its minimum once again.
UK patent application 2,262,123 discloses a spring balance having a tension spring and a torsion spring, in which the adjustment is effected at the bottom, i.e. at the connection to the sliding sash. The return force is increased by way of a ratchet mechanism, and this document discloses means of deactivating the ratchet mechanism if it is necessary or desired to decrease the return force.
UK patent application 2,373,813 discloses a spring balance having a tension spring and a torsion spring, in which adjustment is effected by way of a gearbox connected to the top of the spring balance. The spring balance disclosed in this document has the significant advantage that adjustment can be effected in situ, i.e. without disconnection or removal of the spring balance or the sliding sash. In this spring balance the top of the tension spring is fixed securely to a bracket at the top of the spring balance, which bracket is fixed to the window frame. The bottom of the tension spring carries a block which is connected to the sliding sash and to which the bottom of the torsion spring is also fixed. The top of the torsion spring carries the follower bush through which the spiral rod passes. The top of the spiral rod is fixed to the gearbox at the top of the spring balance. In use, as the spring balance is extended, the bottom of the tension spring is pulled downwardly by way of the block connected to the sliding sash. The torsion spring is also pulled downwardly by the block, and the top of the torsion spring is caused to rotate as the follower bush moves (downwardly) along the spiral rod.
Adjustment of the spring balance of GB 2,373,813 is effected by rotation of the gearbox which causes the top of the spiral rod to rotate, which in turn rotates the top of the torsion spring. Since the bottom of the torsion spring cannot rotate this adjustment affects the preload of the torsion spring and thus the return force of the spring balance.
Whilst the disclosure of GB 2,373,813 avoids the major drawbacks of those spring balances which are adjustable at the bottom, the disclosed spring balance has a significant disadvantage. Specifically, adjustment is effected by rotation of a gear which has a hexagonal head and which is located in a hexagonal opening in the gearbox housing. To effect adjustment the hexagonal head must first be pressed inwardly (against the force of a biasing spring), usefully by a screwdriver, to release the hexagonal head from the hexagonal opening, whereupon the gear can be rotated as desired. However, when the adjustment has been completed the hexagonal head must be aligned with the hexagonal opening before the screwdriver is removed, so that the gear is prevented from rotating. In practice this is very difficult to achieve because the hexagonal head is necessarily a close fit in the hexagonal opening; if the screwdriver is removed with the hexagonal head not engaging the hexagonal opening the torsion spring will rotate freely so as to remove any preload therein. The desired preload must then be reintroduced. Clearly, it will often be necessary to remove all of the preload in the other, unaffected, spring balance at the other side of the window, so that the two spring balances can be adjusted together, and importantly by the same amount.
The difficulty in achieving satisfactory operation is exacerbated by the requirement to push in the screwdriver against the bias of the return spring, and therefore to reduce the pressure upon the screwdriver as this is removed, in order to allow the return spring to force the hexagonal head of the gear towards (and hopefully into) the hexagonal opening. Clearly, as the pressure on the screwdriver is reduced the tendency of the gear to rotate under the influence of the torsion spring is considerable, and even slight misalignment of the hexagonal head will allow the screwdriver to be removed without the head engaging the hexagonal opening, with the consequent free rotation of the torsion spring.
Accordingly, whilst the arrangement of GB 2,373,813 appears simple in using the same gear both to adjust the preload and also to prevent free rotation of the torsion spring, in practice this presents significant difficulties.
Another significant disadvantage of the spring balance of GB 2,373,813 (and which is shared by many prior art spring balances) is that there is no upper limit upon the preload which can be set. It is therefore widely recognised that the spring balances can be over-adjusted. Slight over-adjustment is not too great a concern, though any over-adjustment increases the strain upon the torsion spring (and other components) unnecessarily, leading to a reduction in the useful life of the spring balance. However, significant over-adjustment is a widely-recognised concern, and this can damage a spring balance by exceeding the tolerance either of the torsion spring or other componentry within the spring balance. Typically, significant over-adjustment manifests itself in damage to the follower bush, which is either forcibly separated from the torsion spring, or else damaged so that it rotates freely upon the spiral rod. In both cases the effect of the torsion spring is lost and the spring balance must be extensively repaired or replaced.